Method for producing a colored oxide film on an aluminum or aluminum alloy

ABSTRACT

An aluminum or aluminum alloy coated with a colored oxide film is produced by applying a direct current which changes polarity at a given frequency alternately, to each of two electrodes, one of which is aluminum or aluminum alloy or both of with are aluminum or aluminum alloy, in an electrolytic bath containing a metal salt to form a colored oxide film on one of the electrodes or both of the electrodes.

'United States Patent Kaneda et al. 1 1 July 1, 1975 1 1 METHOD FOR PRODUCING A COLORED 3,634,208 1/1972 Kuroda .5 204/58 OXIDE FILM ON AN LU U 3.6391221 2/1972 Dorsey v 2114/58 1654.100 4/1972 Nagai e1 :11 1 1 1 1 1 204/58 ALUMINUM ALLOY 3,664 932 5/1972 Patrie 1 1 1 1 1 1 204/58 {75] Inventors: Kazuyoshi Kaneda; Yasushi Suzuki, 3,699.856 6/1972 dd /58 both f hi k Kenji wada, 3,717555 211973 Chakavarii et ul, 204/58 Tokyo 3 of Japan 3.751.350 8/1973 Ueki c. 204/58 3.761.362 9/1973 Oida et al. 1 1. 204/58 [73] Assignee: Riken Light Metal Industries Co., 3,773.631 11/1973 lmmel et al 204/58 Japan FOREIGN PATENTS OR APPLICATIONS 1221 Fllcdi June 1, 1973 19 42 3/1971 Japan 204/5s 1211 Appl. No.: 366,207

Primary Examiner-R L. Andrews An ,A r, F- Wt .-,Sh t&N' Foreign Application Priority Data omey gen or "m a as C war 2 lssen June 6, 1972 Japan 47-55574 Nov 13, 1972 Japan 47-112868 [57] ABSTRACT An aluminum or aluminum alloy coated with a col 521 [1.5. CI. 204/ N; 204/58 med Oxide film is Pmduced by applying a direct 51 1 1m. c1 czar 5/00; C23b 9/02 rem which Changes Polarity a given frequency alter 5 Field f Search N 35 N. 56 R nalely. to each of two electrodes, one of which is aluminum or aluminum alloy or both of with are alumi- [56] References Cited num or aluminum alloy in an electrolytic bath con- UNITED STATES PATENTS taining 21 metal salt to form a colored oxide film on one of the electrodes or both of the electrodes. 2930,74] 3/1960 Burger e1 2111 204/228 3,382,160 5/1968 Asada et a1. 1. 204/58 5 Claims, 2 Drawing Figures 0 Ole electrode /7 V 5 sec.

l I U L--60min. 0

Another electrode SHEET 7 1 F I6. I 0 WV 5 One electrode sec. I I l 2 ...60min. /7V 0 W V Another e/ecrrode l u 1 w. 60min. 2,

v'may Ii 3,892,636

SHEET 2 F1612 0 I05 F/rs/ stage /4\/ 1 30min. 1 L 2 2- /2V Essa b Zsec. Second sra 2 2 2 2 f Z J i 2 10min. I /4V 1 METHOD FOR PRODUCING A COLORED OXIDED FILM ON AN ALUMINUM OR ALUMINUM ALLOY The present invention relates to a method for producing a colored oxide film of aluminum or aluminum alloy (referred to as aluninous material hereinafter) and particularly a method for easily producing a colored oxide film having an excellent reproducibility of color tone.

Heretofore, various processes wherein an aluminous material is subjected to an anodic oxidation treatment by applying a direct or alternate current or these currents simultaneously in an electrolytic bath consisting of a solution of a mineral acid, organic acid or a salt of these acids and containing a small amount of a metal salt to adsorb the metal in the metal salt or its compound into an oxide film and to form a colored oxide film showing an inherent color of the metal salt, have been known.

For example, as the alternating current processes, the following processes have been proposed.

a. An organic coloring process of an aluminous material by means of an alternating electrolysis in Japanese Pat. No. 310,401.

b. A process for treating a surface of an aluminous material by means of both alternating current and direct current in Japanese Pat. Application Publication No. 19,842/71.

The direct current processes include the following.

c. A process using a negative current.

d. A process using a direct current intermittently.

However, the above described various processes are not commercially satisfactory and have the following problems to be improved. These problems are (l) improvement of reproducibility of coloration. (2) case of treating operation and the like.

Namely, in these prior arts, the formation of the oxide film and the adsorption of metal ion into the oxide film have a close relation to the other electrolytic conditions and it is relatively difficult to control the electrolytic conditions efficiently and stably by correlating these factors. Accordingly, when the adsorption efficiency of the metal ion is low or the stability of the oxide layer is poor, it is difficult to overcome these drawbacks and determine a practical formation period and control the conditions corresponding to said time.

In some metal salts, it is impossible to obtain desired anodic oxidation coloration.

For obviating these drawbacks the inventors have made various investigations and found that a colored oxide film having an excellent uniformity and a high reproducibility can be easily formed on an aluminous matcrial by effecting an electrolytic treatment by changing the polarity of direct current alternately.

The invention will be explained hereinafter.

An aluminous material to be treated is degreased and washed with water in a conventional manner, for example by degreasing in an aqueous solution of NaOH, washing with water. neutralizing in an aqueous solution of HNO and then washing with water.

Then, the thus treated aluminous material is used as either of two electrodes or both electrodes and an elec trolysis treatment is carried out in an electrolytic bath comprising an aqueous solution of a mineral acid or an orgnaic acid or a salt of these acids, to which one or more metal salts are added, at room temperature by means of a direct current, the polarity of which is periodically reversed. In this case, the direct negative and positive current are periodically reversed. The reversed period is usually within 5 minutes and the voltage to be applied varies depending upon the kind of the metal salt to be added but is generally not more than V whereby a colored oxide film showing an color ihnerent to the above described added metal salt is formed on the aluminous material.

The methods for coloring an aluminous material as described above are as follows.

1. Both the anode and cathode are constituted by an aluminous material and both the electrodes are colored simultaneously.

Firstly, the aluminous materials are degreased and washed with water in the usual manner and the thus treated aluminous materials as such are immersed in an electrolytic bath containing a metal salt as the opposite electrodes.

A direct current having an periodically reversing polarity is applied between both the aluminous material electrodes at a given frequency. In such a manner, on both the aluminous material electrodes, colored oxice films inherent to the above described metal salt are formed.

Since the aluminous materials have been merely degreased and washed with water, when the above described current is applied, an alumina film is formed on the aluminous material anode, while no alumina film is formed on the aluminous material cathode. Then, this state is changed after a given cycle has been passed through and the aluminous material wherein an alumina film has been formed, is changed from the anode to the cathode, while the aluminous material retaining the original state in the cathode is changed from the cathode to the anode.

When the polarity is changed in this manner, the alumina film on the aluminous material which has been changed from the anode to the cathode, is colored by the action of the metal salt, while an alumina film is formed on the aluminous material which is changed from the cathode to the anode, and this alumina film is colored by a metal salt during the next half of the cycle.

As mentioned above, the formation of alumina film and the coloration with a metal salt are repeated alternately, whereby the colored films are formed on both the aluminous material electrodes.

Since both the aluminous material electrodes can be subjected to a coloring oxidation treatment simultaneously, the productivity is improved and further it is merely necessary to effect the degreasing and washing with water of the aluminous material as a pre-treatment and the productivity is further improved from this point.

In this case, the voltages to be applied, the current densities and the changing times of the positive direct current and the negative direct current are equal.

2. As both the anode and cathode, use is made of aluminous materials but on any one of the aluminous materials, a colored oxide film is formed and on the other aluminous material, an oxide film is formed.

In this case, as any one of both the aluminous material electrodes, use is made of an aluminous material in which an oxide film has been already formed and this oxide film coated aluminous material constitutes a cathode and an aluminous material, which has been de greased and washed with water, constitutes the anode. To both the electrodes is applied a direct current which changes polarity alternately in an electrolytic bath containing a metal salt. In such a treatment. the aluminous material previously provided with the oxide film is colored and on the other aluminous material is formed an alumina film.

in this process, even if the metal salt which colors the alumina film. is a metal salt which deposites difficultly on the alumina film. the metal may be easily deposited on the oxide film and the coloration can be effected efflciently.

For example, an aluminous material after being degreased and washed with water is subjected to a usual anodic oxidation treatment to form an oxide film (Alon the aluminous material. This aluminous material is used as an electrode (for example cathode) and as the opposite electrode use is made of an aluminous material which has been degreased and wadhed with water.

Then. to both the aluminous material electrodes is applied a direct current, the polarity of which is changed alternately. in an aqueous electrolytic bath consisting of an aqueous solution of a mineral or organic acid or one or more these salts, to which is added a metal salt. at room temperature. By such an electrolytic treatment. a colored oxide film is formed on the aluminous material provided with a previously formed oxide film and an oxide film is formed on the aluminous material. which has been only degreased and washed with water. Then, the colored aluminous material is taken out from the electrolytic bath and a new aluminous material which has been degreased and washed with water. is put in the electrolytic bath and this aluminous material electrode and the aluminous material electrode coated with an alumina layer are subjected to the same electrolysis treatment as mentioned above. whereby the aluminous material coated with an alumina layer is colored.

In the above described process (2), either one of the electrodes must be an aluminous material previously subjected to an anodic oxidation. at the starting stage. However. even if the aluminous material to be used one electrode has not been separately subjected to an anodic oxidation at the starting stage, the above pro' cess (2) may be carried out.

For example. aluminous materials which have been degreased and washed with water, are arranged as the opposite electrodes in an electrolytic bath, to which a metal salt has been added at the starting stage. Then, between both the electrodes is applied a direct current, the charge transfer of which is different from that in the usual operation. Namely. in the starting stage. a reversing direct current in which the amount of positive charge transferred is larger than the amount of negative charge. is applied in order that the metal is not depos ited on the aluminous material and that only the alu mina film is formed on the aluminous material. Since the positive charge contributes to form the oxide film, its value must be larger than negative. if the negative charge is too large. there is danger that the metal may be deposited on the oxide film.

After the oxide film is formed in one of the aluminous materials. the anodic oxidation is repeated by using the direct current as described above.

In this case. a direct current having a larger negative charge and a smaller positive charge is applied. Namely. the voltage. charging time and current density of the positive current are larger than those ofthe negative current.

3. As one electrode, use is made of an aluminous ma terial wherein an alumina film has been previously formed and as another electrode, use is made of a carbonaceous material or other substance and a direct current which changes the polarity alternately is applied.

An aluminous material which has been already degreased and washed with water. is subjected to a usual anodic oxidation treatment by means of an aqueous solution of sulfuric acid or the like to form a porous oxide film on the surface of the aluminous material.

The aluminous material coated with alumina and the carbonaceous material are subjected to the electrolysis treatment under the same treating conditions, such as the electrolytic bath, currents and the like as described in the above described processes l) and (2). whereby a colored oxide film is formed on the surface of the aluminous material.

Furthermore, in this case a carbonaceous material is used ad the other electrode but as said other electrode use may be made of a previously anodized aluminous material. That is. both the electrodes may be constituted by the aluminous materials treated with anodic oxidation.

4. As one electrode. use is made of an aluminous material which has been degreased and washed with water and not coated with alumina film. and as another elec trode, use is made of a foreign material. for example carbonaceous material.

In this case, firstly a direct current having a larger positive charge and a smaller negative charge is alternately applied to an electrolytic bath containing a metal salt as in the above described processes l 3) to form alumina film the aluminous material. Then a direct current having a smaller positive charge and a larger negative charge is alternately applied to the same bath as described above at a given frequency to form colored oxide film on the aluminous material.

As mentioned above, the methods of the present invention can be carried out as in the above described processes l )(4) and in any case, the oxide film can be colored in an color inherent to the metal salt by passing a direct current which reverses the polarity alternately.

The effects obtained by the present invention are as follows.

Namely, in the usual coloring process by using a metal salt, a porous oxide film has been previously formed on an aluminius material by an anodic oxidation and a metal oxide aluminous deposited on the oxide film by an alternating current. Accordingly, in the conventional process, an oxide film has been previously formed and then the coloration is made by a metal salt by means of an alternating current.

On the contary. according to the present invention, the formation of the oxide film and the coloration owing to a metal salt are alternately or simultaneously effected by means of a direct current which reverses polarity alternately.

For example. as shown in FIG. I, when the aluminous materials are used as both the electrodes. if the aluminous material is used as positive electrode. the alumi' nous material acts as an anode and an oxide film is formed, while if the aluminous material is used as a negative electrode. a metal ion is adsorbed on the oxide film. In this case, both the positive and negative currents which are reversed alternately, are direct current, so, that the reverse period and the voltage to be applied can be established easily and therefore the aluminous material can be colored satisfactorily.

When the coloration is made by the method of the present invention, the method is effective in the follow ing case.

1. When the metal salts which are difficultly depos ited in the oxide film, are added.

2. When the coloring electrolysis time is reduced.

The reason why the reversal period of the direct current is defined as within 5 minutes, is as follows. Even if the positive current is applied for a longer time than the definition, the coloration degree is not improved and this is not advantageous economically and if the negative current is applied for a longer time than the above definition, a redissolution and separation of the oxide film on the aluminous material occurs and the formation of colored oxide film is retarded.

For a better understanding of the invention, reference is made to the accompanying drawings. wherein:

FIG. I shows the wave shape of the direct current used in Example 1; and

FIG. 2 shows the wave shape of the direct current used in Example 4.

The following examples are given for the purpose of illustration of this invention and are not intended as limitations thereof.

EXAMPLE 1 Aluminum alloy 1 100 was degreased by immersing in an 8.0% NaOH aqueous solution kept at 60C for 30 seconds and washed with water. The alloy was further immersed in a 7r HNO aqueous solution kept at room temperature for seconds to effect neutralization and washed with water.

The thus treated aluminum alloy was used as opposite electrodes. The opposite electrodes were connected alternately to a positive direct current and a negative direct current at an alternating time of 5 seconds for minutes in an aqueous solution containing 12.0% by weight of H 50, and 0.0la% by weight of HAuClr4H- O and kept at room temperature to effect an electrolysis of the aluminum alloy. In this case, the voltage of the positive and negative direct currents was kept at 17 V, and the current density thereof was kept at 2 Afdm? As the result, a light reddish purple oxide film was formed on the aluminum alloy.

Another electrolysis was effected for 60 minutes under the same conditions as described above to form a deep reddish purple oxide film having a thickness of 10 ,u

FIG. 1 shows the treating conditions in this Example EXAMPLE 2 Aluminum alloy 6063 was degreased by immersing in an 80% NaOH aqueous solution kept at 60C for 3 minutes. and washed with water. Then the alloy was immersed in a 10% HMO; aqueous solution kept at room temperature for 30 seconds to cffedt neutralization and washed with water.

The thus treated aluminum alloy was used as opposite clectrodes. The opposite electrodes were con nected alternately to a positive direct current and a negatite direct current at an alternating time of 30 secends for 60 minutes in an aqueous solution containing 15.0% by weight of H and 0.00257! by weight of AgNO and kept at room temperature to effect an electrolysis In this case, the voltage of the positive and negative direct currents was kept at 15 V, and the current density thereof was kept at 1.5 A/dm. As the result, a uniform yellowish brown oxide film having a thicknes of 7.5 u was formed on the aluminum alloy.

EXAMPLE 3 The same aluminum alloy as used in Example 1 was degreased and washed with water in the same manner as described in Example I and used as opposite electrodes. The opposite electrodes were connected alternately to a positive direct current and a negative direct current at an alternating time of 10 seconds for 60 minutes in an aqueous solution containing 5.0% by weight of C H O and 0.5% by weight of Na SeO and kept at room temperature to effect an electrolysis. In this case, the voltage of the positive and negative direct currents was kept at 50 V and the current density thereof was kept at 2 A/dm As the result, a uniform orange oxide film having a thickness of 8.0 u was formed on the aluminum alloy.

EXAMPLE 4 The same aluminum alloy as used in Example 1 was degreased and washed with water in the same manner as described in Example 1. The thus treated aluminum alloy was used as one electroce and carbon having a surface area of 2 dm was used as another electrode. The aluminum alloy was subjected to an electrolysis in an aqueous soltuion containing 15% by weight of H 80, and 1.0% by weight of SnSO, and kept at room temperature in the following manner. In first stage, the aluminum alloy electrode was connected to a positive DC voltage of 14 V at a current density of l A/dm for 10 seconds and then connected to a negative DC voltage of 12 V at a current density of 1.5 A/dm for 2 seconds, and such steps were repeated for 30 minutes as shown in FIG. 2a. Then an electrolysis was continued for 10 minutes at a positive direct of 5 V and an alternating time of 2 seconds and at a negative direct current of 14 V and an alternating time of 2 seconds as shown in FIG. 2b. After the first stage treatment, a light amber-colored oxide film was formed on the aluminum alloy, and after the second stage treatment, a deep amber-colored oxide film was formed on the aluminum alloy. FIG. 2 shows the treating condition at the aluminum alloy electrode.

EXAMPLE 5 The same aluminum alloy as used in Example 2, which had been degreased and washed with water in the same manner as described in Example 2, was used as an anode and subjected to a conventional anodic oxidation for 30 minutes in an aqueous soltuion containing 15.0% by weight of H 50 to form a porous oxide film on the aluminum alloy.

Then, the thus treated aluminum alloy was used as one electrode, and carbon was used as another electrode, and an electrolysis of the aluminum alloy was effected in an aqueous containing 10% by weight of H 80, and 0.5% by weight of SnSO and kept at room temperature in the following manner. The aluminum alloy electrode was connected to a positive DC voltage of 3 V at a current density of not higher than 0.1 A/dm for 2 seconds and then connected to a negative DC voltage of i5 V at a current density of l A/dm' for seconds. and such steps were repeated for 30 minutes. As the result. a light amber colored oxide film was formed on the aluminum alloy after 2 minute treatment. a yellowish brown oxide film was formed after 5 minute treatment, a brown oxide film was formed after 10 minute treatment and a black oxide film was formed after 30 minute treatment.

When the above described aluminum alloy coated with porous oxide film was subjected to a cathodic electrolysis (l5 V, l A/dm in the above described electrolytic bath. the oxide film separated within 2-3 minutes. Therefore. it was confirmed that an electrolytic treatment only by the negative current affects adversely the formation of colored oxide film.

EXAMPLE 6 The same aluminum alloy as used in Example l, which was degreased and washed with water in the same manner as described in Example I. was used as an anode. and subjected to an anodic oxidation in the same manner as described in Example 5 to form a porous oxide film on the aluminum alloy.

Then, the thus treated aluminum alloy was used as one electrode. and carbon was used as another electrode. and an electrolysis of the aluminum alloy was cffected in an aqueous solution containing 10.0% by weight of H 50. and 1.0% by weight of CuSO; and kept at room temperature in the following manner. In the positive direct current, the applied voltage was 5 V and the current density was 0.2 A/dm and in the negative direct current. the applied voltage was V and the current density was 2.5 A/dm The alternating time was 2 seconds and the electrolysis time was 15 minutes. A light reddish purple oxide film was formed on the aluminum alloy after 3 minutes treatment. and a brown oxide film was formed after 15 minute treatment.

EXAMPLE 7 The same aluminum alloy as used in Example l was degreased and washed with water in the same manner as described in Example l and the thus treated alloy was used as opposite electrodes. 10% by weight of H 50 0.005% by weight of HAuCl '4H- O and 0.01%

by weight of Ag SO. were added to water and the formed silver chloride was filtered off and the filtrate was used as an electrolytic bath. To the opposite electrodes was applied alternately a positive direct current and a negative direct current at an alternating time of 5 seconds for 60 minutes in the eledtrolytic bath kept at room temperature to effect an electrolysis. In this case. the voltage ofthe positive and negative direct currents was kept at l8 V and the current density thereof was kept at 1.5 A/dm As the result. a reddish brown oxide film having a thickness of 8.0 p. was formed on the aluminum alloy. and it was confirmed that a colored oxide film was obtained by means of a mixture of metal salts.

EXAMPLE 8 Aluminum alloy 1100 or 6063 was degreased and washed with water in a conventional manner. The alloy was used as an anode and subjected to an anodic oxidation for 30 minutes in an aqueous solution of a sulfuric acid to form alumina film having a thickness of about 7 p. on the alloy. Then,the aluminum alloy on which the oxide film had been formed as described above was used as one electrode. and the aluminum alloy which had been only degreased and washed with water was used as another electrode, and an electrolysis treatment was effected under the conditions as shown in the following Table 1. As the result, the aluminum alloy having the previously formed oxide film was colored as shown in Table 1. An oxide film was formed on the aluminum alloy which had been only degreased and washed with waterv The colored aluminum alloy was taken out from the electrolytic bath. and instead of this aluminum alloy. an aluminum alloy which had been degreased and washed with water. was put in the same electrolytic bath as an opposite electrode to the aluminum alloy coated with the oxide film and the electrolytic treatment was made in the same manner as described in the following Table l. Then, the same colored oxide film as that of the above taken out aluminum alloy was formed on the aluminum alloy on which the oxide film had already been formed. and an oxide film was formed on the newly opposed aluminum alloy which had been only degreased and washed with water.

Table l Electrolytic bath Electrolysis condition Example Alter- Elcctro- Color of No. Voltage nating Current lysis oxide film (Aluminum Base solution Added metal salt (V) time density time alloy) (sec) (AIdm (mint 1 H SOplQO wt7r Na. ,WO..2H O:2.5 wtzi 1+) 3 2 (HALE 45 light grcyish (llUU) IR 8 L5 brown I H SO LISU wtr'r K- CT O-;l3.5 wt /l 1+) 4 I ll l-lt" greyish brown (llUO) t-ll7 I ll! 3 H SOMUH wt /i KMnO l .3 wtil 5 2 (I 2 40 gold [6063i ('lili 5 l5 4 C. .H. .O.:7.0 wt7r Nti SeO;.;().' wt i (+l l5 Ill 1 -l 5 so reddish brown lllUUl (-ldl it) 2 5 H. ,SO,: lUtl wt7r NiSOfitl wt'l H) 5 3 02 40 light (M163) l l7 15 l.. yellowish brown s H SO,:l5.ll wtlt SnSOpltl wt: t+l I 0.2 3t) yellowish (E1063) (-1 I X 3 brown 7 H. .SO,:15.() wt? CuSOpUfi wt r l+l 5 Z Ill 31) brovtn l IUU) 51150 3075 wt? l." l Ill 3 3.5 2

not higher 8 H 50 l0.(l wtwi l+l 5 2 than (1i((H;,C'OO). ..-1H;()Zll at? l) l 7U amber color tllUO) H,.BO;.;2 5 W1 1') l0 l0 5 Note lhc \oltage. alternating time and current titlli llt menu the \altlcs applied to the electrode oi aluminum alloy on \khtt'h .io iuidc lihn has prc\iou\ly been formed EXAMPLE 9 Aluminum alloy 6063 was degreased and washed with water in a conventional manner and used as opposite electrodes. A two stage electrolysis treatment was effected in one electrolytic bath by applying a positive direct current and a negative direct current alternately to the opposite electrodes under the following conditions. As the result. a colored oxide film was formed on one aluminum alloy electrode, and an oxide film was formed on another aluminum alloy electrode at the same time.

1. Composition of electrolytic bath:

Base solution: H 50 by weight Added metal salt: SnSO 1.07: by weight 2. Electrolysis condition:

a. First stage electrolysis treatment.

Alternating Current Electrolysis Voltage time density time (VI (secl (A/dm') (min) (H I) 5 2 50 (-l 19 5 2 treatment, oxide films aluminum alloy elec- Alternating Current Electrolysis Voltage time density time IV) (sec) (A/dm (mm.)

In the second stage electrolysis treatment, the oxide film formed on one of the aluminum alloys used as the opposite electrodes was colored yellowish brown while the oxide film formed on another aluminum alloy in the first stage electrolysis treatment maintained its original thickness.

Accordingly, the following treatment was able to be effected in this Example 9 similarly to Example 8. When the colored aluminum alloy was taken out from the electrolytic bath. and an electrolytic treatment was effected in the same electrolytic bath under the same condition as described above by newly opposing the aluminum alloy which had been only degreased and washed with water to the aluminum alloy remaining in the bath, on which the oxide film had been already formed in the above procedure. and oxide film was formed on the newly opposed aluminum alloy. and on another aluminum alloy the signal film already formed was colored yellowish brown. This. aluminum alloy was used as opposite electrodes in one electrolytic bath, whereby formation ofoxide film on the aluminum alloy and coloration of the oxide film were effected alternately.

As described above, the present invention has the following merits.

1. Since electrolysis of aluminous material is effected by applying alternately positive and negative direct currents to the material, a stable oxide film can be formed on the material and at the same time the metal of the added metal salt can be deposited efficiently on the oxide film. Accordingly, a uniformly colored oxide film can be obtained easily and inexpensively with great reducibility.

2. The thickness of the oxide film layer and the depth of color can be varied corresponding to the request of users by combining various values of alternating time, voltage and current density of positive and negative currents alternately applied to aluminous material.

3. Aluminous materials to be electrolytically treated can be used in one electrode or both electrodes. Particularly, when aluminous materials are used in both elec trodes, colored oxide films can be formed inexpensively with high productivity.

4. Metal salts can be added to the base solution alone or in admixture. Accordingly, complicated color tone which has never been obtained in the conventional method can be obtained. This merit as well as that described in the above item 2 is peculiar to the present in vention.

Moreover, even when a metal salt which is difficultly deposited on the oxide film, is added to base solution, the metal can be deposited in a high efficiency.

5. Electrolysis of aluminous material can be effected at both electrodes, whereby formation of oxide film on the aluminous material and coloration of the oxide film can be effected alternately in one electrolytic bath.

6. Stable colored oxide film can be obtained with great reproducibility. Particularly, even when a metal salt which is difficultly deposited on the oxide film is used, a stable colored oxide film can be obtained. Moreover, the color tone of the color oxide film can be more complicated.

7. The treatment described in the above item 5 can be effected efficiently and inexpensively in a commer cial scale.

What is claimed is:

1. Method for producing a colored oxide film on aluminum or an aluminum alloy which comprises arranging a degreased and water-washed aluminum or aluminum alloy, or a material other than aluminum, which can be used as an electrode, as one electrode and a degreased and waterwashed aluminum or aluminum alloy or an alumina film coated aluminum or aluminum alloy as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes a periodically reversing direct current, said current re versing direction at an interval of from 1 second to 5 minutes, the positive charge and the negative charge of said direct current being adjusted by the reversing time, voltage and current density so as to form the colored film on the alumina coated aluminum or aluminum alloy or to form alumina film on the degreased and water-washed aluminum or aluminum alloy.

2. A method for producing colored oxide films on aluminum or an aluminum alloy which comprises arranging a pair of aluminum or aluminum alloy electrodes in an electrolytic hath containing a metal salt and passing between said electrodes a periodically reversing direct current. said current reversing direction at intervals of from I second to 5 minutes at not more than V.

3. A method for producing a colored oxide film on aluminum or an aluminum alloy and alumina film on another aluminum or aluminum alloy which comprises arranging an aluminum or aluminum alloy coated with alumina film as one electrode and an aluminum or aluminum alloy electrode which has been degreased and washed with water as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes such a periodically reversing direct current that a value of negative charge is larger than that of positive charge to form the colored film on the laumina coated aluminum or aluminum alloy and the alumina film on the degreased and water-washed aluminum or aluminum alloy.

4. A method for producing a colored oxide film on aluminum or an aluminum alloy which comprises arranging aluminum or an aluminum alloy coated with alumina film as one electrode and a material other than aluminum as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes such a periodically reversing direct current that the value of negative charge is larger than that of positive charge, to form the colored film on the alumina coated aluminum or aluminum alloy.

5. A method for producing a colored oxide film on aluminum or aluminum alloy which comprises arranging a degreased and water-washed aluminum or aluminum alloy as one electrode and a material other than aluminum as another electrode in an electrolytic bath containing a metal salt and passing between said elec trodes such a periodically reversing direct current that the value of positive charge is larger than that of negative charge, to form alumina film on the aluminum or aluminum alloy electrode and then passing between the thus treated electrodes such a periodically reversing direct current that the value of negative charge is larger than that of positive charge, to form the colored oxide film on the aluminum or aluminum or aluminum alloy electrode. 

1. Method for producing a colored oxide film on aluminum or an aluminum alloy which comprises arranging a degreased and water-washed aluminum or aluminum alloy, or a material other than aluminum, which can be used as an electrode, as one electrode and a degreased and water-washed aluminum or aluminum alloy or an alumina film coated aluminum or aluminum alloy as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes a periodically reversing direct current, said current reversing direction at an interval of from 1 second to 5 minutes, the positive charge and the negative charge of said direct current being adjusted by the reversing time, voltage and current density so as to form the colored film on the alumina coated aluminum or aluminum alloy or to form alumina film on the degreased and water-washed aluminum or aluminum alloy.
 2. A METHOD FOR PRODUCING COLORED OXIDE FILMS ON ALUMNINUM OR AN AUMNUM ALLOY WHICH COMPRISES ARRANGING A PAIR OF ALUMINUM OR ALUMINUM ALLOY ELECTRODES IN AN ELECTROLYTIC BATH CONTAINING A METAL SALT AND PASSING BETWEEN SAID ELECTRODES A PERIODICALLY REVERSING DIRECT CURRENT, SAID CURRENT REVERSING DIRECTION AT INTERVALS OF FROM 1 SECOND TO 5 MINUTES AT NOT MORE THAN 70V.
 3. A method for producing a colored oxide film on aluminum or an aluminum alloy and alumina film on another aluminum or aluminum alloy which comprises arranging an aluminum or aluminum alloy coated with alumina film as one electrode and an aluminum or aluminum alloy electrode which has been degreased and washed with water as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes such a periodically reversing direct current that a value of negative charge is larger than that of positive charge to form the colored film on the laumina coated aluminum or aluminum alloy and the alumina film on the degreased and water-washed aluminum or aluminum alloy.
 4. A method for producing a colored oxide film on aluminum or an aluminum alloy which comprises arranging aluminum or an aluminum alloy coated with alumina film as one electrode and a material other than aluminum as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes such a periodically reversing direct current that the value of negative charge is larger than that of positive charge, to form the colored film on the alumina coated aluminum or aluminum alloy.
 5. A method for producing a coloRed oxide film on aluminum or aluminum alloy which comprises arranging a degreased and water-washed aluminum or aluminum alloy as one electrode and a material other than aluminum as another electrode in an electrolytic bath containing a metal salt and passing between said electrodes such a periodically reversing direct current that the value of positive charge is larger than that of negative charge, to form alumina film on the aluminum or aluminum alloy electrode and then passing between the thus treated electrodes such a periodically reversing direct current that the value of negative charge is larger than that of positive charge, to form the colored oxide film on the aluminum or aluminum or aluminum alloy electrode. 